Long Term Evolution (LTE) is an evolution of the third generation (3G) mobile telecommunication technology, and is initiated from the 3GPP (3rd Generation Partnership Project) Conference held in Toronto, 2004. It's a popular misconception that LTE is the fourth generation (4G) mobile telecommunication technology. Actually, LTE, as a transition between 3G and 4G, is a global standard of 3.9G. LTE improves and enhances the radio interface technology of 3G by using Orthogonal Frequency Division Multiplexing (OFDM) and Multiple-Input Multiple-Output (MIMO) as its only standard for wireless network evolution. Under a 20 MHz spectrum bandwidth, LTE provides a downlink peak rate of 326 Mbit/s and an uplink peak rate of 86 Mbit/s, so that performances of cell-edge users are improved, cell capacity is increased and system delay is decreased.
With the development of the LET technology and the acceleration of the commercialization process of LET communication terminals (including cell phones, wireless network cards and modules), the communication terminals should be automatically calibrated in order to meet requirements of mass production of LTE terminals. The automatic calibration at least includes Automatic Power Control (APC) calibration, Automatic Gain Control (AGC) calibration and Automatic Frequency Control (AFC) calibration, where AFC is an automatic control method for maintaining a fixed relationship between an output signal frequency of a communication terminal and a predetermined frequency.
In development and production processes of LET communication terminals, frequency accuracy of frequency synthesizers should be calibrated, so as to keep synchronization with the base station. In such way, network searching period after booting may be shortened, especially when tracking frequency in a high or low temperature environment.
A common process for calibrating frequency synthesizers in thousands of communication terminals on the production line may include: a communication terminal transmitting a modulated signal of an associated communication mode to an integrated tester; the integrated tester demodulating the modulated signal in the physic layer, where the modulated signal may have a modulation mode like Globe System of Mobile Communication (GSM), Time Division-Synchronous Code Division Multiple Access (TD-SCDMA), Wideband Code Division Multiple Access (WCDMA), etc., and the demodulated results includes information of frequency error; and calculating frequency characteristics of a reference crystal oscillator of the communication terminal based on the information of frequency error; and storing calibration parameters, which include a center oscillation frequency point and a frequency calibration step, in a memory of the communication terminal. Therefore, in daily use, the communication terminal can extract these calibrated parameters to find a network, keep the clock synchronized with the base station and so on.
However, with development of the communication technology and gradual evolution of the mobile telecommunication along 2G, 3G and 4G because communication mode of communication terminals is changed by using a new generation mobile telecommunication technology, equipment and software in R&D and product calibration lines cannot be used to calibrate frequency synthesizers in the communication terminals with the new communication mode. For example, if equipments originally used for calibrating frequency synthesizers in TD-SCDMA or WCDMA communication terminals are used for calibrating frequency synthesizers in LTE communication terminals, original R&D and product calibration lines should be updated, or equipment and software having LTE digital modem modules should be purchased, which results in increasing of frequency calibration cost.